The claimed invention relates generally to disc drive data storage devices and more particularly to a novel overmold used to support a coil of an acutator used to move an array of read/write heads adjacent a corresponding number of disc recording surfaces.
Data storage devices of the type known as xe2x80x9cWinchesterxe2x80x9d disc drives are well known in the industry. These disc drives magnetically record digital data on circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a spindle motor. In disc drives of the current generation, the discs are rotated at speeds of up to 10,000 revolutions per minute.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably positioned by an actuator assembly. Each head typically includes electromagnetic transducer read and write elements which are carried on an air bearing slider. The slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly each head in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the heads and the discs, the heads are attached to and supported by head suspensions or flexures.
Historically, the actuator assembly used to position the heads has assumed many forms. Most disc drives of the current generation incorporate an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator includes a pivot shaft fixed to a disc drive housing base deck at a location adjacent an outer edge of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies. The actuator bearing housing supports a coil that is immersed in a magnetic field of permanent magnets. The permanent magnets are mounted to the disc drive housing base deck.
The actuator assembly also includes a centrally located E-block. On the side of the actuator bearing housing opposite the coil are a plurality of vertically aligned, radially extending actuator arms. The actuator arms and the coil are connected to the E-block. The flexures are mounted to the actuator arms. When current is applied to the coil, the coil produces a magnetic field that interacts with the magnetic field of the permanent magnets to pivot the actuator bearing housing about the pivot shaft, thereby moving the heads across the disc surfaces.
A closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to maintain the head position with respect to the data tracks. Such a servo system obtains head position information from servo data fields written to the tracks during disc drive manufacturing to maintain a selected head over an associated track during a track following mode of operation.
A seek mode of operation, which comprises the initial acceleration of a head away from an initial track and the subsequent deceleration of the head toward a destination track, is also controlled by the servo system. Such seek operations are typically velocity controlled, in that the velocity of the head is repetitively measured and compared to a velocity profile, with the current applied to the coil being generally proportional to the difference between the actual and profile velocities as the head is moved toward the destination track.
A continuing trend in the industry is to provide disc drives with ever increasing data storage and transfer capabilities. This trend has led to efforts to minimize the time required to perform a seek operation. A typical seek operation includes an initial overhead time during which the disc drive services its internal operations, a seek time during which the head is moved to and settled on the destination track, and a latency time during which the drive waits until a particular sector on the destination track reaches the head as the disc rotates relative to the heads.
Seek times have been minimized through the application of large amounts of current to the coil during acceleration and deceleration phases of a seek operation. However, as the current is increased, the electrical energy dissipated as heat by the coil also increases so that the temperature of the coil increases. As the coil temperature increases, the resistance of the coil increases and the magnitude of the control current is limited, which also adversely affects the seek time. Additionally, elevated coil temperatures can cause degradation of adhesive and insulating materials used in the construction of a voice coil motor (VCM).
Actuator overmolds have been used in disc drives of the existing art to support the coil and to dampen vibration. An actuator overmold is typically composed of liquid crystal polymer (LCP), thermoplastic, or thermoset plastic resin formed over portions of the actuator assembly. The actuator body, coil and other components are typically assembled, placed inside a mold having a mold cavity, and a thermoplastic is injected in the mold cavity.
A problem has arisen during a seek operation with some models because the heat generated by the coil causes the coil to become hot and expand. The heat generated by the coil heats the overmold which also expands, but at a different rate than the rate at which the coils expands. Following the seek operation, the coil and the overmold contract, also at different rates. As a result of the differential rates of expansion and contraction, stresses have developed between the coil and the overmold. These stresses are relieved suddenly by a xe2x80x9cpoppingxe2x80x9d which causes a sudden force to be internally generated within the actuator. This force can cause the heads to move xe2x80x9coff trackxe2x80x9d and thereby caused data read and write errors.
The stresses caused by the differential rate of expansion are further exacerbated by the fact that glass/mineral filled polymer resins in an amorphous state exhibit a sudden increase in their coefficient of thermal expansion when their temperature reaches a glass transition temperature. The glass transition temperature marks the incipiency of relative motion between polymer chains causing them to store internal strain energy if the polymer is constrained in some manner. This change in the coefficient of thermal expansion is reversible unless the entire polymer becomes crystalline. Upon cooling of the overmold material, the stored energy becomes available when the temperature becomes lower than the glass transition temperature and results in popping.
Thus, there is a need for an improved actuator design that overcomes these and other limitations of the prior art.
In accordance with preferred embodiments, a thermally conducting and electrically insulating overmold is provided to encapsulate and support an actuator coil of a disc drive actuator assembly. The overmold is made from a base resin mixed with a ceramic filler compound of boron nitride, alumina, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, chromium oxide or some combination thereof. In some preferred embodiments, the base resin is a thermoplastic such as a liquid crystal polymer, polyethylene naphthalate, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyphthalamide, polypheneline sulfide, polycarbonates, polyetheretheketones and polyphenyline oxide. In some embodiments, the overmold material includes a coupling agent and/or glass fiber.
The overmold material is preferably selected to be electrically insulative and thermally conductive, and to have a rate of thermal expansion that substantially matches that of the actuator coil to reduce head position errors due to popping (sudden force induced by the thermal expansion and contraction of the actuator coil).
These and various other features as well as advantages which characterize the claimed invention will become apparent upon reading the following detailed description and upon reviewing the associated drawings.